1. Field of the Invention
This invention relates to lubricant compositions having improved antioxidant characteristics. More particularly, the invention relates to lubricants to which have been added a combination of additives which imparts antioxidants properties thereto.
2. Discussion of the Prior Art
Lubricating oils and greases are subject to oxidative deterioration at elevated temperatures or upon prolonged exposure in an ordinary atmosphere. Such deterioration is in many instances evidenced by an increase in acidity and in viscosity, and when the deterioration is severe enough it can cause metal parts to corrode. Additionally, there is a loss of lubricating properties which may lead to complete failure of the part or parts in contact with the oil. Numerous additives have been tried as antioxidants in lubricants, but many of them are only marginally effective except at the higher concentrations.
The use of amines as lubricant antioxidants is, of course, well known. As an example, U.S. Pat. No. 3,126,344 discloses the use of such amines as phenyl-.alpha.-naphthylamine, p-amino diphenylamine and p,p'-dioctyldiphenylamine as antioxidants in synthetic ester lubricating oils. Representative of other patents disclosing the use of single amines are U.S. Pat. No. 3,231,499 and U.S. Pat. No. 3,309,314.
In addition, it is known to use mixtures of amines as antioxidants for lubricants. U.S. Pat. No. 3,148,147 discloses for instance, the use of a mixture of phenyl-.alpha.-naphthylamine and diphenylamine. In U.S. Pat. No. 3,247,111 it is taught that a particularly outstanding combination involves a mixture of phenyl-.alpha.-naphthylamine and dioctyl diphenylamine. U.S. Pat. No. 3,309,318 teaches the same combination.
As will appear hereinafter, the combinations of the present invention are superior to the mixtures taught by U.S. Pat. No. 3,247,111 and 3,309,318.